One concept for increasing the capacity of optical storage media is to use holographic data storage. In this case the whole volume of the holographic storage medium is used for storing information, not just a few layers as for conventional optical storage media. In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam is modulated by a spatial light modulator and carries the information to be recorded in the form of data pages. One advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc.
Data storage on a holographic storage medium requires a very precise servo concept with regard to focus, track and tangential direction, in order to be able to write or read a hologram at a stable position. For a holographic disk a servo format similar to a DVD (Digital Versatile Disk) could be used for the servo function. For example, in EP1310952 Horimai et al. disclose the concept of having a substrate similar to a DVD as a guiding structure underneath a holographic disk medium. In this case a servo light beam is focused onto the holographic disk medium with the same objective lens as the holographic beam, i.e. the light beam used for holographic recording or readout. When the holographic and the servo beams are fixed relatively to each other, the servo beam can act as a reference for the beam used for holographic recording. The servo beam is focused onto the guiding structure, whereas the holographic beam is focused as appropriate for the chosen holographic recording process.
With the DVD servo format every track has two neighboring tracks at a distance of 0.74 μm. This repetitive groove and land change causes a diffraction effect, which perturbs the reading and writing operation. In addition, the current optical designs of holographic storage systems require a minimum distance of at least 10 μm from one hologram to the next when information is stored as data pages. This means that after each rotation of the holographic disk a track jump over multiple tracks has to take place. This makes a real continuous recording impossible and complicates the addressing of the holograms.